Modern looms conventionally include so-called drop wires for detecting the breakage or other loss of tension in individual warp threads being delivered to the loom for weaving with transversely inserted filling threads. Typically, the drop wires take the form of elongated wires or thin plates having an aperture or other means for engagement thereby of an individual warp thread. These drop wires essentially hang loosely upon the individual warp threads and upon the loss of tension therein slip or fall downwardly under the influence of gravity to complete an electrical circuit providing an indication of the occurrence of a break or other disturbance in the normal tensioned condition of that thread which if allowed to go uncorrected would result in the creation of a perceptible defect in the ultimately woven fabric when the thread discontinuity finally reaches the weaving zone. In addition, the occurrence of a break or extensive slack in the warp yarns can result in more or less serious entanglement of the loose or broken end of the thread with adjacent warp threads so as to impede the passage of such entangled threads through the harness motion, leading in serious cases to damage to the harness motion or other major problems. In the usual arrangement, the output signal from the drop wire detector serves to automatically halt the weaving operation by activating a so-called loom stop motion. While the drop wire arrangement is generally effective to detect warp thread breakage and the like, such wires are necessarily in physical engagement with each individual thread and while they can be constructed of thin lightweight material, there nevertheless results some consequential abrasion upon the threads which may be undesirable especially in the case of very delicate threads and the like. Aside from the aspect of possible thread wear, the drop wire arrangement is in any case inconvenient since each warp thread must be manipulated individually into engagement with the corresponding drop wire which significantly increases the difficulty of and time required for threading up of the warp in the loom preparatory to the beginning of weaving.
According to a description of the prior art appearing in U.S. Pat. No. 3,902,534, it has been previously proposed in the art to provide photoelectric detection means projecting adjacent the underside of the warp for detecting the passage of a broken thread across the light beam thereof under the influence of gravity so as to avoid the necessity for physical contact with the warp threads themselves. However, the patent explains that since the warp threads are under a significant amount of tension, particularly when the harnesses of the harness motion are in maximum separated position to form the warp shed for insertion of the filling thread therethrough, the breakage of a thread may well result in the thread entangled with adjacent warp threads and thereby precluded from falling under the force of gravity into the path of the photoelectric beam and to provide a more positive mode of detection, this patent combines with the photoelectric thread detection means a wire brush or comb which wipes against the warp array across the width thereof at a locus spaced from the photoelectric beam so as to engage and entrap the end of any broken or loose warp thread and upon continued rotation of the brush or comb, positively draw the thus-engaged thread across the path of the photoelectric beam to intercept the same and activate the photoelectric receiving element. As an alternative to the photoelectric detector, other kinds of thread detectors can be substituted such as a wire extending across the warp and connected to a sensitive microswitch to sense the presence of any warp thread deviated from the normal warp path by engagement with the wiping element.
This combination while perhaps avoiding the problem inherent in any detection system dependent upon the action of gravity to displace the thread to be detected obviously achieves this improvement at the price of physical engagement of the threads by the wiping brush or comb with the consequential re-creation of the same basic problem as that accompanying the use of lease rods which the photoelectric approach endeavored to eliminate.
Therefore, the objective of achieving positive reliable warp thread detection without the necessity for any physical contact with the warp threads themselves remain unachieved in this art.